An Application of Terrain and Environmental Modelling in a Large-scale Forestry Experiment

Within-site heterogeneity, particularly of soil and climatic variables is a difficulty facing forestry researchers in the design of field trials. Forest soils are notoriously variable and small changes in soil properties can have a large impact on tree productivity. Likewise, within-site variation in microclimate is known to influence growth. Within a particular climatic envelope, this variation is controlled to a large extent by topography. In this study, a range of terrain-derived attributes and indices was calculated for each of 40 growth plots from a digital elevation model (DEM) in an attempt to quantify the effects of spatial heterogeneity and establish whether they improved the interpretation of data from a field experiment. In addition, selected soil profile measurements were analyzed for their impact on tree growth and correlated with calculated terrain attributes. The presence of gleying (evidence of waterlogging) in the profile accounted for a significant proportion of the residual variation in growth response after treatment effects were removed. A combination of predicted erosion index (ei), relative available soil water (raswc), dynamic wetness index (dynwet) and tangential curvature (tancurve) contributed to significant improvements in model predictions of growth. A physiologically based model of tree growth (ProMod) was applied to the site to model the effect of these variables on tree growth. We found that by using a terrain-derived waterlogging or gleying variable as input, ProMod could successfully account for variation in tree growth due to waterlogging (gleying) and some additional between-plot variation in growth due to soil depth.